Merge branch 'ps/history-split'

"git history" learned the "split" subcommand.

* ps/history-split:
  builtin/history: implement "split" subcommand
  builtin/history: split out extended function to create commits
  cache-tree: allow writing in-memory index as tree
  add-patch: allow disabling editing of hunks
  add-patch: add support for in-memory index patching
  add-patch: remove dependency on "add-interactive" subsystem
  add-patch: split out `struct interactive_options`
  add-patch: split out header from "add-interactive.h"

16 files changed, 1592 insertions, 341 deletions

diff --git a/Documentation/git-history.adoc b/Documentation/git-history.adoc
index cc019de697..24dc907033 100644
--- a/Documentation/git-history.adoc
+++ b/Documentation/git-history.adoc
@@ -9,6 +9,7 @@ SYNOPSIS
 --------
 [synopsis]
 git history reword <commit> [--dry-run] [--update-refs=(branches|head)]
+git history split <commit> [--dry-run] [--update-refs=(branches|head)] [--] [<pathspec>...]
 
 DESCRIPTION
 -----------
@@ -57,6 +58,26 @@ The following commands are available to rewrite history in different ways:
 	details of this commit remain unchanged. This command will spawn an
 	editor with the current message of that commit.
 
+`split <commit> [--] [<pathspec>...]`::
+	Interactively split up <commit> into two commits by choosing
+	hunks introduced by it that will be moved into the new split-out
+	commit. These hunks will then be written into a new commit that
+	becomes the parent of the previous commit. The original commit
+	stays intact, except that its parent will be the newly split-out
+	commit.
++
+The commit messages of the split-up commits will be asked for by launching
+the configured editor. Authorship of the commit will be the same as for the
+original commit.
++
+If passed, _<pathspec>_ can be used to limit which changes shall be split out
+of the original commit. Files not matching any of the pathspecs will remain
+part of the original commit. For more details, see the 'pathspec' entry in
+linkgit:gitglossary[7].
++
+It is invalid to select either all or no hunks, as that would lead to
+one of the commits becoming empty.
+
 OPTIONS
 -------
 
@@ -72,6 +93,47 @@ OPTIONS
 	descendants of the original commit will be rewritten. With `head`, only
 	the current `HEAD` reference will be rewritten. Defaults to `branches`.
 
+EXAMPLES
+--------
+
+Split a commit
+~~~~~~~~~~~~~~
+
+----------
+$ git log --stat --oneline
+3f81232 (HEAD -> main) original
+ bar | 1 +
+ foo | 1 +
+ 2 files changed, 2 insertions(+)
+
+$ git history split HEAD
+diff --git a/bar b/bar
+new file mode 100644
+index 0000000..5716ca5
+--- /dev/null
++++ b/bar
+@@ -0,0 +1 @@
++bar
+(1/1) Stage addition [y,n,q,a,d,p,?]? y
+
+diff --git a/foo b/foo
+new file mode 100644
+index 0000000..257cc56
+--- /dev/null
++++ b/foo
+@@ -0,0 +1 @@
++foo
+(1/1) Stage addition [y,n,q,a,d,p,?]? n
+
+$ git log --stat --oneline
+7cebe64 (HEAD -> main) original
+ foo | 1 +
+ 1 file changed, 1 insertion(+)
+d1582f3 split-out commit
+ bar | 1 +
+ 1 file changed, 1 insertion(+)
+----------
+
 GIT
 ---
 Part of the linkgit:git[1] suite
diff --git a/add-interactive.c b/add-interactive.c
index 1580639682..3cf8a1dbf8 100644
--- a/add-interactive.c
+++ b/add-interactive.c
@@ -3,7 +3,6 @@
 #include "git-compat-util.h"
 #include "add-interactive.h"
 #include "color.h"
-#include "config.h"
 #include "diffcore.h"
 #include "gettext.h"
 #include "hash.h"
@@ -20,120 +19,18 @@
 #include "prompt.h"
 #include "tree.h"
 
-static void init_color(struct repository *r, enum git_colorbool use_color,
-		       const char *section_and_slot, char *dst,
-		       const char *default_color)
-{
-	char *key = xstrfmt("color.%s", section_and_slot);
-	const char *value;
-
-	if (!want_color(use_color))
-		dst[0] = '\0';
-	else if (repo_config_get_value(r, key, &value) ||
-		 color_parse(value, dst))
-		strlcpy(dst, default_color, COLOR_MAXLEN);
-
-	free(key);
-}
-
-static enum git_colorbool check_color_config(struct repository *r, const char *var)
-{
-	const char *value;
-	enum git_colorbool ret;
-
-	if (repo_config_get_value(r, var, &value))
-		ret = GIT_COLOR_UNKNOWN;
-	else
-		ret = git_config_colorbool(var, value);
-
-	/*
-	 * Do not rely on want_color() to fall back to color.ui for us. It uses
-	 * the value parsed by git_color_config(), which may not have been
-	 * called by the main command.
-	 */
-	if (ret == GIT_COLOR_UNKNOWN &&
-	    !repo_config_get_value(r, "color.ui", &value))
-		ret = git_config_colorbool("color.ui", value);
-
-	return ret;
-}
-
 void init_add_i_state(struct add_i_state *s, struct repository *r,
-		      struct add_p_opt *add_p_opt)
+		      struct interactive_options *opts)
 {
 	s->r = r;
-	s->context = -1;
-	s->interhunkcontext = -1;
-	s->auto_advance = add_p_opt->auto_advance;
-
-	s->use_color_interactive = check_color_config(r, "color.interactive");
-
-	init_color(r, s->use_color_interactive, "interactive.header",
-		   s->header_color, GIT_COLOR_BOLD);
-	init_color(r, s->use_color_interactive, "interactive.help",
-		   s->help_color, GIT_COLOR_BOLD_RED);
-	init_color(r, s->use_color_interactive, "interactive.prompt",
-		   s->prompt_color, GIT_COLOR_BOLD_BLUE);
-	init_color(r, s->use_color_interactive, "interactive.error",
-		   s->error_color, GIT_COLOR_BOLD_RED);
-	strlcpy(s->reset_color_interactive,
-		want_color(s->use_color_interactive) ? GIT_COLOR_RESET : "", COLOR_MAXLEN);
-
-	s->use_color_diff = check_color_config(r, "color.diff");
-
-	init_color(r, s->use_color_diff, "diff.frag", s->fraginfo_color,
-		   diff_get_color(s->use_color_diff, DIFF_FRAGINFO));
-	init_color(r, s->use_color_diff, "diff.context", s->context_color,
-		   "fall back");
-	if (!strcmp(s->context_color, "fall back"))
-		init_color(r, s->use_color_diff, "diff.plain",
-			   s->context_color,
-			   diff_get_color(s->use_color_diff, DIFF_CONTEXT));
-	init_color(r, s->use_color_diff, "diff.old", s->file_old_color,
-		   diff_get_color(s->use_color_diff, DIFF_FILE_OLD));
-	init_color(r, s->use_color_diff, "diff.new", s->file_new_color,
-		   diff_get_color(s->use_color_diff, DIFF_FILE_NEW));
-	strlcpy(s->reset_color_diff,
-		want_color(s->use_color_diff) ? GIT_COLOR_RESET : "", COLOR_MAXLEN);
-
-	FREE_AND_NULL(s->interactive_diff_filter);
-	repo_config_get_string(r, "interactive.difffilter",
-			       &s->interactive_diff_filter);
-
-	FREE_AND_NULL(s->interactive_diff_algorithm);
-	repo_config_get_string(r, "diff.algorithm",
-			       &s->interactive_diff_algorithm);
-
-	if (!repo_config_get_int(r, "diff.context", &s->context))
-		if (s->context < 0)
-			die(_("%s cannot be negative"), "diff.context");
-	if (!repo_config_get_int(r, "diff.interHunkContext", &s->interhunkcontext))
-		if (s->interhunkcontext < 0)
-			die(_("%s cannot be negative"), "diff.interHunkContext");
-
-	repo_config_get_bool(r, "interactive.singlekey", &s->use_single_key);
-	if (s->use_single_key)
-		setbuf(stdin, NULL);
-
-	if (add_p_opt->context != -1) {
-		if (add_p_opt->context < 0)
-			die(_("%s cannot be negative"), "--unified");
-		s->context = add_p_opt->context;
-	}
-	if (add_p_opt->interhunkcontext != -1) {
-		if (add_p_opt->interhunkcontext < 0)
-			die(_("%s cannot be negative"), "--inter-hunk-context");
-		s->interhunkcontext = add_p_opt->interhunkcontext;
-	}
+	interactive_config_init(&s->cfg, r, opts);
 }
 
 void clear_add_i_state(struct add_i_state *s)
 {
-	FREE_AND_NULL(s->interactive_diff_filter);
-	FREE_AND_NULL(s->interactive_diff_algorithm);
+	interactive_config_clear(&s->cfg);
 	memset(s, 0, sizeof(*s));
-	s->use_color_interactive = GIT_COLOR_UNKNOWN;
-	s->use_color_diff = GIT_COLOR_UNKNOWN;
+	interactive_config_clear(&s->cfg);
 }
 
 /*
@@ -287,7 +184,7 @@ static void list(struct add_i_state *s, struct string_list *list, int *selected,
 		return;
 
 	if (opts->header)
-		color_fprintf_ln(stdout, s->header_color,
+		color_fprintf_ln(stdout, s->cfg.header_color,
 				 "%s", opts->header);
 
 	for (i = 0; i < list->nr; i++) {
@@ -355,7 +252,7 @@ static ssize_t list_and_choose(struct add_i_state *s,
 
 		list(s, &items->items, items->selected, &opts->list_opts);
 
-		color_fprintf(stdout, s->prompt_color, "%s", opts->prompt);
+		color_fprintf(stdout, s->cfg.prompt_color, "%s", opts->prompt);
 		fputs(singleton ? "> " : ">> ", stdout);
 		fflush(stdout);
 
@@ -433,7 +330,7 @@ static ssize_t list_and_choose(struct add_i_state *s,
 
 			if (from < 0 || from >= items->items.nr ||
 			    (singleton && from + 1 != to)) {
-				color_fprintf_ln(stderr, s->error_color,
+				color_fprintf_ln(stderr, s->cfg.error_color,
 						 _("Huh (%s)?"), p);
 				break;
 			} else if (singleton) {
@@ -993,7 +890,7 @@ static int run_patch(struct add_i_state *s, const struct pathspec *ps,
 				free(files->items.items[i].string);
 			} else if (item->index.unmerged ||
 				 item->worktree.unmerged) {
-				color_fprintf_ln(stderr, s->error_color,
+				color_fprintf_ln(stderr, s->cfg.error_color,
 						 _("ignoring unmerged: %s"),
 						 files->items.items[i].string);
 				free(item);
@@ -1015,10 +912,10 @@ static int run_patch(struct add_i_state *s, const struct pathspec *ps,
 	opts->prompt = N_("Patch update");
 	count = list_and_choose(s, files, opts);
 	if (count > 0) {
-		struct add_p_opt add_p_opt = {
-			.context = s->context,
-			.interhunkcontext = s->interhunkcontext,
-			.auto_advance = s->auto_advance
+		struct interactive_options opts = {
+			.context = s->cfg.context,
+			.interhunkcontext = s->cfg.interhunkcontext,
+			.auto_advance = s->cfg.auto_advance,
 		};
 		struct strvec args = STRVEC_INIT;
 		struct pathspec ps_selected = { 0 };
@@ -1030,7 +927,7 @@ static int run_patch(struct add_i_state *s, const struct pathspec *ps,
 		parse_pathspec(&ps_selected,
 			       PATHSPEC_ALL_MAGIC & ~PATHSPEC_LITERAL,
 			       PATHSPEC_LITERAL_PATH, "", args.v);
-		res = run_add_p(s->r, ADD_P_ADD, &add_p_opt, NULL, &ps_selected);
+		res = run_add_p(s->r, ADD_P_ADD, &opts, NULL, &ps_selected, 0);
 		strvec_clear(&args);
 		clear_pathspec(&ps_selected);
 	}
@@ -1066,10 +963,10 @@ static int run_diff(struct add_i_state *s, const struct pathspec *ps,
 		struct child_process cmd = CHILD_PROCESS_INIT;
 
 		strvec_pushl(&cmd.args, "git", "diff", "-p", "--cached", NULL);
-		if (s->context != -1)
-			strvec_pushf(&cmd.args, "--unified=%i", s->context);
-		if (s->interhunkcontext != -1)
-			strvec_pushf(&cmd.args, "--inter-hunk-context=%i", s->interhunkcontext);
+		if (s->cfg.context != -1)
+			strvec_pushf(&cmd.args, "--unified=%i", s->cfg.context);
+		if (s->cfg.interhunkcontext != -1)
+			strvec_pushf(&cmd.args, "--inter-hunk-context=%i", s->cfg.interhunkcontext);
 		strvec_pushl(&cmd.args, oid_to_hex(!is_initial ? &oid :
 			     s->r->hash_algo->empty_tree), "--", NULL);
 		for (i = 0; i < files->items.nr; i++)
@@ -1087,17 +984,17 @@ static int run_help(struct add_i_state *s, const struct pathspec *ps UNUSED,
 		    struct prefix_item_list *files UNUSED,
 		    struct list_and_choose_options *opts UNUSED)
 {
-	color_fprintf_ln(stdout, s->help_color, "status        - %s",
+	color_fprintf_ln(stdout, s->cfg.help_color, "status        - %s",
 			 _("show paths with changes"));
-	color_fprintf_ln(stdout, s->help_color, "update        - %s",
+	color_fprintf_ln(stdout, s->cfg.help_color, "update        - %s",
 			 _("add working tree state to the staged set of changes"));
-	color_fprintf_ln(stdout, s->help_color, "revert        - %s",
+	color_fprintf_ln(stdout, s->cfg.help_color, "revert        - %s",
 			 _("revert staged set of changes back to the HEAD version"));
-	color_fprintf_ln(stdout, s->help_color, "patch         - %s",
+	color_fprintf_ln(stdout, s->cfg.help_color, "patch         - %s",
 			 _("pick hunks and update selectively"));
-	color_fprintf_ln(stdout, s->help_color, "diff          - %s",
+	color_fprintf_ln(stdout, s->cfg.help_color, "diff          - %s",
 			 _("view diff between HEAD and index"));
-	color_fprintf_ln(stdout, s->help_color, "add untracked - %s",
+	color_fprintf_ln(stdout, s->cfg.help_color, "add untracked - %s",
 			 _("add contents of untracked files to the staged set of changes"));
 
 	return 0;
@@ -1105,21 +1002,21 @@ static int run_help(struct add_i_state *s, const struct pathspec *ps UNUSED,
 
 static void choose_prompt_help(struct add_i_state *s)
 {
-	color_fprintf_ln(stdout, s->help_color, "%s",
+	color_fprintf_ln(stdout, s->cfg.help_color, "%s",
 			 _("Prompt help:"));
-	color_fprintf_ln(stdout, s->help_color, "1          - %s",
+	color_fprintf_ln(stdout, s->cfg.help_color, "1          - %s",
 			 _("select a single item"));
-	color_fprintf_ln(stdout, s->help_color, "3-5        - %s",
+	color_fprintf_ln(stdout, s->cfg.help_color, "3-5        - %s",
 			 _("select a range of items"));
-	color_fprintf_ln(stdout, s->help_color, "2-3,6-9    - %s",
+	color_fprintf_ln(stdout, s->cfg.help_color, "2-3,6-9    - %s",
 			 _("select multiple ranges"));
-	color_fprintf_ln(stdout, s->help_color, "foo        - %s",
+	color_fprintf_ln(stdout, s->cfg.help_color, "foo        - %s",
 			 _("select item based on unique prefix"));
-	color_fprintf_ln(stdout, s->help_color, "-...       - %s",
+	color_fprintf_ln(stdout, s->cfg.help_color, "-...       - %s",
 			 _("unselect specified items"));
-	color_fprintf_ln(stdout, s->help_color, "*          - %s",
+	color_fprintf_ln(stdout, s->cfg.help_color, "*          - %s",
 			 _("choose all items"));
-	color_fprintf_ln(stdout, s->help_color, "           - %s",
+	color_fprintf_ln(stdout, s->cfg.help_color, "           - %s",
 			 _("(empty) finish selecting"));
 }
 
@@ -1154,7 +1051,7 @@ static void print_command_item(int i, int selected UNUSED,
 
 static void command_prompt_help(struct add_i_state *s)
 {
-	const char *help_color = s->help_color;
+	const char *help_color = s->cfg.help_color;
 	color_fprintf_ln(stdout, help_color, "%s", _("Prompt help:"));
 	color_fprintf_ln(stdout, help_color, "1          - %s",
 			 _("select a numbered item"));
@@ -1165,7 +1062,7 @@ static void command_prompt_help(struct add_i_state *s)
 }
 
 int run_add_i(struct repository *r, const struct pathspec *ps,
-	      struct add_p_opt *add_p_opt)
+	      struct interactive_options *interactive_opts)
 {
 	struct add_i_state s = { NULL };
 	struct print_command_item_data data = { "[", "]" };
@@ -1208,15 +1105,15 @@ int run_add_i(struct repository *r, const struct pathspec *ps,
 			->util = util;
 	}
 
-	init_add_i_state(&s, r, add_p_opt);
+	init_add_i_state(&s, r, interactive_opts);
 
 	/*
 	 * When color was asked for, use the prompt color for
 	 * highlighting, otherwise use square brackets.
 	 */
-	if (want_color(s.use_color_interactive)) {
-		data.color = s.prompt_color;
-		data.reset = s.reset_color_interactive;
+	if (want_color(s.cfg.use_color_interactive)) {
+		data.color = s.cfg.prompt_color;
+		data.reset = s.cfg.reset_color_interactive;
 	}
 	print_file_item_data.color = data.color;
 	print_file_item_data.reset = data.reset;
diff --git a/add-interactive.h b/add-interactive.h
index 7843397775..eefa2edc7c 100644
--- a/add-interactive.h
+++ b/add-interactive.h
@@ -1,57 +1,21 @@
 #ifndef ADD_INTERACTIVE_H
 #define ADD_INTERACTIVE_H
 
-#include "color.h"
+#include "add-patch.h"
 
-struct add_p_opt {
-	int context;
-	int interhunkcontext;
-	int auto_advance;
-};
-
-#define ADD_P_OPT_INIT { .context = -1, .interhunkcontext = -1, .auto_advance = 1 }
+struct pathspec;
+struct repository;
 
 struct add_i_state {
 	struct repository *r;
-	enum git_colorbool use_color_interactive;
-	enum git_colorbool use_color_diff;
-	char header_color[COLOR_MAXLEN];
-	char help_color[COLOR_MAXLEN];
-	char prompt_color[COLOR_MAXLEN];
-	char error_color[COLOR_MAXLEN];
-	char reset_color_interactive[COLOR_MAXLEN];
-
-	char fraginfo_color[COLOR_MAXLEN];
-	char context_color[COLOR_MAXLEN];
-	char file_old_color[COLOR_MAXLEN];
-	char file_new_color[COLOR_MAXLEN];
-	char reset_color_diff[COLOR_MAXLEN];
-
-	int use_single_key;
-	char *interactive_diff_filter, *interactive_diff_algorithm;
-	int context, interhunkcontext;
-	int auto_advance;
+	struct interactive_config cfg;
 };
 
 void init_add_i_state(struct add_i_state *s, struct repository *r,
-		      struct add_p_opt *add_p_opt);
+		      struct interactive_options *opts);
 void clear_add_i_state(struct add_i_state *s);
 
-struct repository;
-struct pathspec;
 int run_add_i(struct repository *r, const struct pathspec *ps,
-	      struct add_p_opt *add_p_opt);
-
-enum add_p_mode {
-	ADD_P_ADD,
-	ADD_P_STASH,
-	ADD_P_RESET,
-	ADD_P_CHECKOUT,
-	ADD_P_WORKTREE,
-};
-
-int run_add_p(struct repository *r, enum add_p_mode mode,
-	      struct add_p_opt *o, const char *revision,
-	      const struct pathspec *ps);
+	      struct interactive_options *opts);
 
 #endif
diff --git a/add-patch.c b/add-patch.c
index 8c03f710d3..4e28e5c187 100644
--- a/add-patch.c
+++ b/add-patch.c
@@ -2,11 +2,15 @@
 #define DISABLE_SIGN_COMPARE_WARNINGS
 
 #include "git-compat-util.h"
-#include "add-interactive.h"
+#include "add-patch.h"
 #include "advice.h"
+#include "commit.h"
+#include "config.h"
+#include "diff.h"
 #include "editor.h"
 #include "environment.h"
 #include "gettext.h"
+#include "hex.h"
 #include "object-name.h"
 #include "pager.h"
 #include "read-cache-ll.h"
@@ -260,7 +264,10 @@ struct hunk {
 };
 
 struct add_p_state {
-	struct add_i_state s;
+	struct repository *r;
+	struct index_state *index;
+	const char *index_file;
+	struct interactive_config cfg;
 	struct strbuf answer, buf;
 
 	/* parsed diff */
@@ -278,6 +285,123 @@ struct add_p_state {
 	const char *revision;
 };
 
+static void init_color(struct repository *r,
+		       enum git_colorbool use_color,
+		       const char *section_and_slot, char *dst,
+		       const char *default_color)
+{
+	char *key = xstrfmt("color.%s", section_and_slot);
+	const char *value;
+
+	if (!want_color(use_color))
+		dst[0] = '\0';
+	else if (repo_config_get_value(r, key, &value) ||
+		 color_parse(value, dst))
+		strlcpy(dst, default_color, COLOR_MAXLEN);
+
+	free(key);
+}
+
+static enum git_colorbool check_color_config(struct repository *r, const char *var)
+{
+	const char *value;
+	enum git_colorbool ret;
+
+	if (repo_config_get_value(r, var, &value))
+		ret = GIT_COLOR_UNKNOWN;
+	else
+		ret = git_config_colorbool(var, value);
+
+	/*
+	 * Do not rely on want_color() to fall back to color.ui for us. It uses
+	 * the value parsed by git_color_config(), which may not have been
+	 * called by the main command.
+	 */
+	if (ret == GIT_COLOR_UNKNOWN &&
+	    !repo_config_get_value(r, "color.ui", &value))
+		ret = git_config_colorbool("color.ui", value);
+
+	return ret;
+}
+
+void interactive_config_init(struct interactive_config *cfg,
+			     struct repository *r,
+			     struct interactive_options *opts)
+{
+	cfg->context = -1;
+	cfg->interhunkcontext = -1;
+	cfg->auto_advance = opts->auto_advance;
+
+	cfg->use_color_interactive = check_color_config(r, "color.interactive");
+
+	init_color(r, cfg->use_color_interactive, "interactive.header",
+		   cfg->header_color, GIT_COLOR_BOLD);
+	init_color(r, cfg->use_color_interactive, "interactive.help",
+		   cfg->help_color, GIT_COLOR_BOLD_RED);
+	init_color(r, cfg->use_color_interactive, "interactive.prompt",
+		   cfg->prompt_color, GIT_COLOR_BOLD_BLUE);
+	init_color(r, cfg->use_color_interactive, "interactive.error",
+		   cfg->error_color, GIT_COLOR_BOLD_RED);
+	strlcpy(cfg->reset_color_interactive,
+		want_color(cfg->use_color_interactive) ? GIT_COLOR_RESET : "", COLOR_MAXLEN);
+
+	cfg->use_color_diff = check_color_config(r, "color.diff");
+
+	init_color(r, cfg->use_color_diff, "diff.frag", cfg->fraginfo_color,
+		   diff_get_color(cfg->use_color_diff, DIFF_FRAGINFO));
+	init_color(r, cfg->use_color_diff, "diff.context", cfg->context_color,
+		   "fall back");
+	if (!strcmp(cfg->context_color, "fall back"))
+		init_color(r, cfg->use_color_diff, "diff.plain",
+			   cfg->context_color,
+			   diff_get_color(cfg->use_color_diff, DIFF_CONTEXT));
+	init_color(r, cfg->use_color_diff, "diff.old", cfg->file_old_color,
+		diff_get_color(cfg->use_color_diff, DIFF_FILE_OLD));
+	init_color(r, cfg->use_color_diff, "diff.new", cfg->file_new_color,
+		diff_get_color(cfg->use_color_diff, DIFF_FILE_NEW));
+	strlcpy(cfg->reset_color_diff,
+		want_color(cfg->use_color_diff) ? GIT_COLOR_RESET : "", COLOR_MAXLEN);
+
+	FREE_AND_NULL(cfg->interactive_diff_filter);
+	repo_config_get_string(r, "interactive.difffilter",
+			       &cfg->interactive_diff_filter);
+
+	FREE_AND_NULL(cfg->interactive_diff_algorithm);
+	repo_config_get_string(r, "diff.algorithm",
+			       &cfg->interactive_diff_algorithm);
+
+	if (!repo_config_get_int(r, "diff.context", &cfg->context))
+		if (cfg->context < 0)
+			die(_("%s cannot be negative"), "diff.context");
+	if (!repo_config_get_int(r, "diff.interHunkContext", &cfg->interhunkcontext))
+		if (cfg->interhunkcontext < 0)
+			die(_("%s cannot be negative"), "diff.interHunkContext");
+
+	repo_config_get_bool(r, "interactive.singlekey", &cfg->use_single_key);
+	if (cfg->use_single_key)
+		setbuf(stdin, NULL);
+
+	if (opts->context != -1) {
+		if (opts->context < 0)
+			die(_("%s cannot be negative"), "--unified");
+		cfg->context = opts->context;
+	}
+	if (opts->interhunkcontext != -1) {
+		if (opts->interhunkcontext < 0)
+			die(_("%s cannot be negative"), "--inter-hunk-context");
+		cfg->interhunkcontext = opts->interhunkcontext;
+	}
+}
+
+void interactive_config_clear(struct interactive_config *cfg)
+{
+	FREE_AND_NULL(cfg->interactive_diff_filter);
+	FREE_AND_NULL(cfg->interactive_diff_algorithm);
+	memset(cfg, 0, sizeof(*cfg));
+	cfg->use_color_interactive = GIT_COLOR_UNKNOWN;
+	cfg->use_color_diff = GIT_COLOR_UNKNOWN;
+}
+
 static void add_p_state_clear(struct add_p_state *s)
 {
 	size_t i;
@@ -289,7 +413,7 @@ static void add_p_state_clear(struct add_p_state *s)
 	for (i = 0; i < s->file_diff_nr; i++)
 		free(s->file_diff[i].hunk);
 	free(s->file_diff);
-	clear_add_i_state(&s->s);
+	interactive_config_clear(&s->cfg);
 }
 
 __attribute__((format (printf, 2, 3)))
@@ -298,9 +422,9 @@ static void err(struct add_p_state *s, const char *fmt, ...)
 	va_list args;
 
 	va_start(args, fmt);
-	fputs(s->s.error_color, stdout);
+	fputs(s->cfg.error_color, stdout);
 	vprintf(fmt, args);
-	puts(s->s.reset_color_interactive);
+	puts(s->cfg.reset_color_interactive);
 	va_end(args);
 }
 
@@ -318,7 +442,7 @@ static void setup_child_process(struct add_p_state *s,
 
 	cp->git_cmd = 1;
 	strvec_pushf(&cp->env,
-		     INDEX_ENVIRONMENT "=%s", s->s.r->index_file);
+		     INDEX_ENVIRONMENT "=%s", s->index_file);
 }
 
 static int parse_range(const char **p,
@@ -423,12 +547,12 @@ static int parse_diff(struct add_p_state *s, const struct pathspec *ps)
 	int res;
 
 	strvec_pushv(&args, s->mode->diff_cmd);
-	if (s->s.context != -1)
-		strvec_pushf(&args, "--unified=%i", s->s.context);
-	if (s->s.interhunkcontext != -1)
-		strvec_pushf(&args, "--inter-hunk-context=%i", s->s.interhunkcontext);
-	if (s->s.interactive_diff_algorithm)
-		strvec_pushf(&args, "--diff-algorithm=%s", s->s.interactive_diff_algorithm);
+	if (s->cfg.context != -1)
+		strvec_pushf(&args, "--unified=%i", s->cfg.context);
+	if (s->cfg.interhunkcontext != -1)
+		strvec_pushf(&args, "--inter-hunk-context=%i", s->cfg.interhunkcontext);
+	if (s->cfg.interactive_diff_algorithm)
+		strvec_pushf(&args, "--diff-algorithm=%s", s->cfg.interactive_diff_algorithm);
 	if (s->revision) {
 		struct object_id oid;
 		strvec_push(&args,
@@ -457,9 +581,9 @@ static int parse_diff(struct add_p_state *s, const struct pathspec *ps)
 	}
 	strbuf_complete_line(plain);
 
-	if (want_color_fd(1, s->s.use_color_diff)) {
+	if (want_color_fd(1, s->cfg.use_color_diff)) {
 		struct child_process colored_cp = CHILD_PROCESS_INIT;
-		const char *diff_filter = s->s.interactive_diff_filter;
+		const char *diff_filter = s->cfg.interactive_diff_filter;
 
 		setup_child_process(s, &colored_cp, NULL);
 		xsnprintf((char *)args.v[color_arg_index], 8, "--color");
@@ -692,7 +816,7 @@ static void render_hunk(struct add_p_state *s, struct hunk *hunk,
 				   hunk->colored_end - hunk->colored_start);
 			return;
 		} else {
-			strbuf_addstr(out, s->s.fraginfo_color);
+			strbuf_addstr(out, s->cfg.fraginfo_color);
 			p = s->colored.buf + header->colored_extra_start;
 			len = header->colored_extra_end
 				- header->colored_extra_start;
@@ -714,7 +838,7 @@ static void render_hunk(struct add_p_state *s, struct hunk *hunk,
 		if (len)
 			strbuf_add(out, p, len);
 		else if (colored)
-			strbuf_addf(out, "%s\n", s->s.reset_color_diff);
+			strbuf_addf(out, "%s\n", s->cfg.reset_color_diff);
 		else
 			strbuf_addch(out, '\n');
 	}
@@ -1103,12 +1227,12 @@ static void recolor_hunk(struct add_p_state *s, struct hunk *hunk)
 
 		strbuf_addstr(&s->colored,
 			      plain[current] == '-' ?
-			      s->s.file_old_color :
+			      s->cfg.file_old_color :
 			      plain[current] == '+' ?
-			      s->s.file_new_color :
-			      s->s.context_color);
+			      s->cfg.file_new_color :
+			      s->cfg.context_color);
 		strbuf_add(&s->colored, plain + current, eol - current);
-		strbuf_addstr(&s->colored, s->s.reset_color_diff);
+		strbuf_addstr(&s->colored, s->cfg.reset_color_diff);
 		if (next > eol)
 			strbuf_add(&s->colored, plain + eol, next - eol);
 		current = next;
@@ -1237,7 +1361,7 @@ static int run_apply_check(struct add_p_state *s,
 
 static int read_single_character(struct add_p_state *s)
 {
-	if (s->s.use_single_key) {
+	if (s->cfg.use_single_key) {
 		int res = read_key_without_echo(&s->answer);
 		printf("%s\n", res == EOF ? "" : s->answer.buf);
 		return res;
@@ -1251,7 +1375,7 @@ static int read_single_character(struct add_p_state *s)
 static int prompt_yesno(struct add_p_state *s, const char *prompt)
 {
 	for (;;) {
-		color_fprintf(stdout, s->s.prompt_color, "%s", _(prompt));
+		color_fprintf(stdout, s->cfg.prompt_color, "%s", _(prompt));
 		fflush(stdout);
 		if (read_single_character(s) == EOF)
 			return -1;
@@ -1439,7 +1563,7 @@ static void apply_patch(struct add_p_state *s, struct file_diff *file_diff)
 		strbuf_reset(&s->buf);
 		reassemble_patch(s, file_diff, 0, &s->buf);
 
-		discard_index(s->s.r->index);
+		discard_index(s->index);
 		if (s->mode->apply_for_checkout)
 			apply_for_checkout(s, &s->buf,
 					s->mode->is_reverse);
@@ -1450,9 +1574,11 @@ static void apply_patch(struct add_p_state *s, struct file_diff *file_diff)
 					NULL, 0, NULL, 0))
 				error(_("'git apply' failed"));
 		}
-		if (repo_read_index(s->s.r) >= 0)
-			repo_refresh_and_write_index(s->s.r, REFRESH_QUIET, 0,
-							1, NULL, NULL, NULL);
+		if (read_index_from(s->index, s->index_file, s->r->gitdir) >= 0 &&
+		    s->index == s->r->index) {
+			repo_refresh_and_write_index(s->r, REFRESH_QUIET, 0,
+						     1, NULL, NULL, NULL);
+		}
 	}
 
 }
@@ -1478,7 +1604,9 @@ static bool get_first_undecided(const struct file_diff *file_diff, size_t *idx)
 	return false;
 }
 
-static size_t patch_update_file(struct add_p_state *s, size_t idx)
+static size_t patch_update_file(struct add_p_state *s,
+				size_t idx,
+				unsigned flags)
 {
 	size_t hunk_index = 0;
 	ssize_t i, undecided_previous, undecided_next, rendered_hunk_index = -1;
@@ -1540,7 +1668,7 @@ static size_t patch_update_file(struct add_p_state *s, size_t idx)
 		/* Everything decided? */
 		if (undecided_previous < 0 && undecided_next < 0 &&
 		    hunk->use != UNDECIDED_HUNK) {
-				if (!s->s.auto_advance)
+				if (!s->cfg.auto_advance)
 					all_decided = 1;
 				else {
 					patch_update_resp++;
@@ -1589,16 +1717,17 @@ static size_t patch_update_file(struct add_p_state *s, size_t idx)
 				permitted |= ALLOW_SPLIT;
 				strbuf_addstr(&s->buf, ",s");
 			}
-			if (hunk_index + 1 > file_diff->mode_change &&
+			if (!(flags & ADD_P_DISALLOW_EDIT) &&
+			    hunk_index + 1 > file_diff->mode_change &&
 			    !file_diff->deleted) {
 				permitted |= ALLOW_EDIT;
 				strbuf_addstr(&s->buf, ",e");
 			}
-			if (!s->s.auto_advance && s->file_diff_nr > 1) {
+			if (!s->cfg.auto_advance && s->file_diff_nr > 1) {
 				permitted |= ALLOW_GOTO_NEXT_FILE;
 				strbuf_addstr(&s->buf, ",>");
 			}
-			if (!s->s.auto_advance && s->file_diff_nr > 1) {
+			if (!s->cfg.auto_advance && s->file_diff_nr > 1) {
 				permitted |= ALLOW_GOTO_PREVIOUS_FILE;
 				strbuf_addstr(&s->buf, ",<");
 			}
@@ -1613,7 +1742,7 @@ static size_t patch_update_file(struct add_p_state *s, size_t idx)
 		else
 			prompt_mode_type = PROMPT_HUNK;
 
-		printf("%s(%"PRIuMAX"/%"PRIuMAX") ", s->s.prompt_color,
+		printf("%s(%"PRIuMAX"/%"PRIuMAX") ", s->cfg.prompt_color,
 			      (uintmax_t)hunk_index + 1,
 			      (uintmax_t)(file_diff->hunk_nr
 						? file_diff->hunk_nr
@@ -1626,8 +1755,8 @@ static size_t patch_update_file(struct add_p_state *s, size_t idx)
 		}
 		printf(_(s->mode->prompt_mode[prompt_mode_type]),
 			hunk_use_decision, s->buf.buf);
-		if (*s->s.reset_color_interactive)
-			fputs(s->s.reset_color_interactive, stdout);
+		if (*s->cfg.reset_color_interactive)
+			fputs(s->cfg.reset_color_interactive, stdout);
 		fflush(stdout);
 		if (read_single_character(s) == EOF) {
 			patch_update_resp = s->file_diff_nr;
@@ -1678,7 +1807,7 @@ soft_increment:
 		} else if (ch == 'q') {
 			patch_update_resp = s->file_diff_nr;
 			break;
-		} else if (!s->s.auto_advance && s->answer.buf[0] == '>') {
+		} else if (!s->cfg.auto_advance && s->answer.buf[0] == '>') {
 			if (permitted & ALLOW_GOTO_NEXT_FILE) {
 				if (patch_update_resp == s->file_diff_nr - 1)
 					patch_update_resp = 0;
@@ -1689,7 +1818,7 @@ soft_increment:
 				err(s, _("No next file"));
 				continue;
 			}
-		} else if (!s->s.auto_advance && s->answer.buf[0] == '<') {
+		} else if (!s->cfg.auto_advance && s->answer.buf[0] == '<') {
 			if (permitted & ALLOW_GOTO_PREVIOUS_FILE) {
 				if (patch_update_resp == 0)
 					patch_update_resp = s->file_diff_nr - 1;
@@ -1812,7 +1941,7 @@ soft_increment:
 				err(s, _("Sorry, cannot split this hunk"));
 			} else if (!split_hunk(s, file_diff,
 					     hunk - file_diff->hunk)) {
-				color_fprintf_ln(stdout, s->s.header_color,
+				color_fprintf_ln(stdout, s->cfg.header_color,
 						 _("Split into %d hunks."),
 						 (int)splittable_into);
 				rendered_hunk_index = -1;
@@ -1830,7 +1959,7 @@ soft_increment:
 		} else if (s->answer.buf[0] == '?') {
 			const char *p = _(help_patch_remainder), *eol = p;
 
-			color_fprintf(stdout, s->s.help_color, "%s",
+			color_fprintf(stdout, s->cfg.help_color, "%s",
 				      _(s->mode->help_patch_text));
 
 			/*
@@ -1854,13 +1983,13 @@ soft_increment:
 						if (file_diff->hunk[i].use == SKIP_HUNK)
 							skipped += 1;
 					}
-					color_fprintf_ln(stdout, s->s.help_color, _(p),
+					color_fprintf_ln(stdout, s->cfg.help_color, _(p),
 							 total, used, skipped);
 				}
 				if (*p != '?' && !strchr(s->buf.buf, *p))
 					continue;
 
-				color_fprintf_ln(stdout, s->s.help_color,
+				color_fprintf_ln(stdout, s->cfg.help_color,
 						 "%.*s", (int)(eol - p), p);
 			}
 		} else {
@@ -1869,23 +1998,62 @@ soft_increment:
 		}
 	}
 
-	if (s->s.auto_advance)
+	if (s->cfg.auto_advance)
 		apply_patch(s, file_diff);
 
 	putchar('\n');
 	return patch_update_resp;
 }
 
+static int run_add_p_common(struct add_p_state *state,
+			    const struct pathspec *ps,
+			    unsigned flags)
+{
+	size_t binary_count = 0;
+	size_t i;
+
+	if (parse_diff(state, ps) < 0)
+		return -1;
+
+	for (i = 0; i < state->file_diff_nr;) {
+		if (state->file_diff[i].binary && !state->file_diff[i].hunk_nr) {
+			binary_count++;
+			i++;
+			continue;
+		}
+		if ((i = patch_update_file(state, i, flags)) == state->file_diff_nr)
+			break;
+	}
+
+	if (!state->cfg.auto_advance)
+		for (i = 0; i < state->file_diff_nr; i++)
+			apply_patch(state, state->file_diff + i);
+
+	if (state->file_diff_nr == 0)
+		err(state, _("No changes."));
+	else if (binary_count == state->file_diff_nr)
+		err(state, _("Only binary files changed."));
+
+	return 0;
+}
+
 int run_add_p(struct repository *r, enum add_p_mode mode,
-	      struct add_p_opt *o, const char *revision,
-	      const struct pathspec *ps)
+	      struct interactive_options *opts, const char *revision,
+	      const struct pathspec *ps,
+	      unsigned flags)
 {
 	struct add_p_state s = {
-		{ r }, STRBUF_INIT, STRBUF_INIT, STRBUF_INIT, STRBUF_INIT
+		.r = r,
+		.index = r->index,
+		.index_file = r->index_file,
+		.answer = STRBUF_INIT,
+		.buf = STRBUF_INIT,
+		.plain = STRBUF_INIT,
+		.colored = STRBUF_INIT,
 	};
-	size_t i, binary_count = 0;
+	int ret;
 
-	init_add_i_state(&s.s, r, o);
+	interactive_config_init(&s.cfg, r, opts);
 
 	if (mode == ADD_P_STASH)
 		s.mode = &patch_mode_stash;
@@ -1916,30 +2084,91 @@ int run_add_p(struct repository *r, enum add_p_mode mode,
 	if (repo_read_index(r) < 0 ||
 	    (!s.mode->index_only &&
 	     repo_refresh_and_write_index(r, REFRESH_QUIET, 0, 1,
-					  NULL, NULL, NULL) < 0) ||
-	    parse_diff(&s, ps) < 0) {
-		add_p_state_clear(&s);
-		return -1;
+					  NULL, NULL, NULL) < 0)) {
+		ret = -1;
+		goto out;
 	}
 
-	for (i = 0; i < s.file_diff_nr;) {
-		if (s.file_diff[i].binary && !s.file_diff[i].hunk_nr) {
-			binary_count++;
-			i++;
-			continue;
-		}
-		 if ((i = patch_update_file(&s, i)) == s.file_diff_nr)
-			break;
-    }
-	if (!s.s.auto_advance)
-		for (i = 0; i < s.file_diff_nr; i++)
-			apply_patch(&s, s.file_diff + i);
+	ret = run_add_p_common(&s, ps, flags);
+	if (ret < 0)
+		goto out;
 
-	if (s.file_diff_nr == 0)
-		err(&s, _("No changes."));
-	else if (binary_count == s.file_diff_nr)
-		err(&s, _("Only binary files changed."));
+	ret = 0;
 
+out:
 	add_p_state_clear(&s);
-	return 0;
+	return ret;
+}
+
+int run_add_p_index(struct repository *r,
+		    struct index_state *index,
+		    const char *index_file,
+		    struct interactive_options *opts,
+		    const char *revision,
+		    const struct pathspec *ps,
+		    unsigned flags)
+{
+	struct patch_mode mode = {
+		.apply_args = { "--cached", NULL },
+		.apply_check_args = { "--cached", NULL },
+		.prompt_mode = {
+			N_("Stage mode change [y,n,q,a,d%s,?]? "),
+			N_("Stage deletion [y,n,q,a,d%s,?]? "),
+			N_("Stage addition [y,n,q,a,d%s,?]? "),
+			N_("Stage this hunk [y,n,q,a,d%s,?]? ")
+		},
+		.edit_hunk_hint = N_("If the patch applies cleanly, the edited hunk "
+				     "will immediately be marked for staging."),
+		.help_patch_text =
+			N_("y - stage this hunk\n"
+			   "n - do not stage this hunk\n"
+			   "q - quit; do not stage this hunk or any of the remaining "
+				"ones\n"
+			   "a - stage this hunk and all later hunks in the file\n"
+			   "d - do not stage this hunk or any of the later hunks in "
+				"the file\n"),
+		.index_only = 1,
+	};
+	struct add_p_state s = {
+		.r = r,
+		.index = index,
+		.index_file = index_file,
+		.answer = STRBUF_INIT,
+		.buf = STRBUF_INIT,
+		.plain = STRBUF_INIT,
+		.colored = STRBUF_INIT,
+		.mode = &mode,
+		.revision = revision,
+	};
+	char parent_tree_oid[GIT_MAX_HEXSZ + 1];
+	struct commit *commit;
+	int ret;
+
+	interactive_config_init(&s.cfg, r, opts);
+
+	commit = lookup_commit_reference_by_name(revision);
+	if (!commit) {
+		err(&s, _("Revision does not refer to a commit"));
+		ret = -1;
+		goto out;
+	}
+
+	if (commit->parents)
+		oid_to_hex_r(parent_tree_oid, get_commit_tree_oid(commit->parents->item));
+	else
+		oid_to_hex_r(parent_tree_oid, r->hash_algo->empty_tree);
+
+	mode.diff_cmd[0] = "diff-tree";
+	mode.diff_cmd[1] = "-r";
+	mode.diff_cmd[2] = parent_tree_oid;
+
+	ret = run_add_p_common(&s, ps, flags);
+	if (ret < 0)
+		goto out;
+
+	ret = 0;
+
+out:
+	add_p_state_clear(&s);
+	return ret;
 }
diff --git a/add-patch.h b/add-patch.h
new file mode 100644
index 0000000000..fb6d975b68
--- /dev/null
+++ b/add-patch.h
@@ -0,0 +1,74 @@
+#ifndef ADD_PATCH_H
+#define ADD_PATCH_H
+
+#include "color.h"
+
+struct index_state;
+struct pathspec;
+struct repository;
+
+struct interactive_options {
+	int context;
+	int interhunkcontext;
+	int auto_advance;
+};
+
+#define INTERACTIVE_OPTIONS_INIT { \
+	.context = -1, \
+	.interhunkcontext = -1, \
+	.auto_advance = 1, \
+}
+
+struct interactive_config {
+	enum git_colorbool use_color_interactive;
+	enum git_colorbool use_color_diff;
+	char header_color[COLOR_MAXLEN];
+	char help_color[COLOR_MAXLEN];
+	char prompt_color[COLOR_MAXLEN];
+	char error_color[COLOR_MAXLEN];
+	char reset_color_interactive[COLOR_MAXLEN];
+
+	char fraginfo_color[COLOR_MAXLEN];
+	char context_color[COLOR_MAXLEN];
+	char file_old_color[COLOR_MAXLEN];
+	char file_new_color[COLOR_MAXLEN];
+	char reset_color_diff[COLOR_MAXLEN];
+
+	int use_single_key;
+	char *interactive_diff_filter, *interactive_diff_algorithm;
+	int context, interhunkcontext;
+	int auto_advance;
+};
+
+void interactive_config_init(struct interactive_config *cfg,
+			     struct repository *r,
+			     struct interactive_options *opts);
+void interactive_config_clear(struct interactive_config *cfg);
+
+enum add_p_mode {
+	ADD_P_ADD,
+	ADD_P_STASH,
+	ADD_P_RESET,
+	ADD_P_CHECKOUT,
+	ADD_P_WORKTREE,
+};
+
+enum add_p_flags {
+	/* Disallow "editing" hunks. */
+	ADD_P_DISALLOW_EDIT = (1 << 0),
+};
+
+int run_add_p(struct repository *r, enum add_p_mode mode,
+	      struct interactive_options *opts, const char *revision,
+	      const struct pathspec *ps,
+	      unsigned flags);
+
+int run_add_p_index(struct repository *r,
+		    struct index_state *index,
+		    const char *index_file,
+		    struct interactive_options *opts,
+		    const char *revision,
+		    const struct pathspec *ps,
+		    unsigned flags);
+
+#endif
diff --git a/builtin/add.c b/builtin/add.c
index 0ee21692c2..7737ab878b 100644
--- a/builtin/add.c
+++ b/builtin/add.c
@@ -31,7 +31,7 @@ static const char * const builtin_add_usage[] = {
 	NULL
 };
 static int patch_interactive, add_interactive, edit_interactive;
-static struct add_p_opt add_p_opt = ADD_P_OPT_INIT;
+static struct interactive_options interactive_opts = INTERACTIVE_OPTIONS_INIT;
 static int take_worktree_changes;
 static int add_renormalize;
 static int pathspec_file_nul;
@@ -160,7 +160,7 @@ static int refresh(struct repository *repo, int verbose, const struct pathspec *
 int interactive_add(struct repository *repo,
 		    const char **argv,
 		    const char *prefix,
-		    int patch, struct add_p_opt *add_p_opt)
+		    int patch, struct interactive_options *interactive_opts)
 {
 	struct pathspec pathspec;
 	int ret;
@@ -172,9 +172,9 @@ int interactive_add(struct repository *repo,
 		       prefix, argv);
 
 	if (patch)
-		ret = !!run_add_p(repo, ADD_P_ADD, add_p_opt, NULL, &pathspec);
+		ret = !!run_add_p(repo, ADD_P_ADD, interactive_opts, NULL, &pathspec, 0);
 	else
-		ret = !!run_add_i(repo, &pathspec, add_p_opt);
+		ret = !!run_add_i(repo, &pathspec, interactive_opts);
 
 	clear_pathspec(&pathspec);
 	return ret;
@@ -256,10 +256,10 @@ static struct option builtin_add_options[] = {
 	OPT_GROUP(""),
 	OPT_BOOL('i', "interactive", &add_interactive, N_("interactive picking")),
 	OPT_BOOL('p', "patch", &patch_interactive, N_("select hunks interactively")),
-	OPT_BOOL(0, "auto-advance", &add_p_opt.auto_advance,
+	OPT_BOOL(0, "auto-advance", &interactive_opts.auto_advance,
 		 N_("auto advance to the next file when selecting hunks interactively")),
-	OPT_DIFF_UNIFIED(&add_p_opt.context),
-	OPT_DIFF_INTERHUNK_CONTEXT(&add_p_opt.interhunkcontext),
+	OPT_DIFF_UNIFIED(&interactive_opts.context),
+	OPT_DIFF_INTERHUNK_CONTEXT(&interactive_opts.interhunkcontext),
 	OPT_BOOL('e', "edit", &edit_interactive, N_("edit current diff and apply")),
 	OPT__FORCE(&ignored_too, N_("allow adding otherwise ignored files"), 0),
 	OPT_BOOL('u', "update", &take_worktree_changes, N_("update tracked files")),
@@ -402,9 +402,9 @@ int cmd_add(int argc,
 	prepare_repo_settings(repo);
 	repo->settings.command_requires_full_index = 0;
 
-	if (add_p_opt.context < -1)
+	if (interactive_opts.context < -1)
 		die(_("'%s' cannot be negative"), "--unified");
-	if (add_p_opt.interhunkcontext < -1)
+	if (interactive_opts.interhunkcontext < -1)
 		die(_("'%s' cannot be negative"), "--inter-hunk-context");
 
 	if (patch_interactive)
@@ -414,13 +414,13 @@ int cmd_add(int argc,
 			die(_("options '%s' and '%s' cannot be used together"), "--dry-run", "--interactive/--patch");
 		if (pathspec_from_file)
 			die(_("options '%s' and '%s' cannot be used together"), "--pathspec-from-file", "--interactive/--patch");
-		exit(interactive_add(repo, argv + 1, prefix, patch_interactive, &add_p_opt));
+		exit(interactive_add(repo, argv + 1, prefix, patch_interactive, &interactive_opts));
 	} else {
-		if (add_p_opt.context != -1)
+		if (interactive_opts.context != -1)
 			die(_("the option '%s' requires '%s'"), "--unified", "--interactive/--patch");
-		if (add_p_opt.interhunkcontext != -1)
+		if (interactive_opts.interhunkcontext != -1)
 			die(_("the option '%s' requires '%s'"), "--inter-hunk-context", "--interactive/--patch");
-		if (!add_p_opt.auto_advance)
+		if (!interactive_opts.auto_advance)
 			die(_("the option '%s' requires '%s'"), "--no-auto-advance", "--interactive/--patch");
 	}
 
diff --git a/builtin/checkout.c b/builtin/checkout.c
index 1d1667fa4c..e031e61886 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c
@@ -532,7 +532,7 @@ static int checkout_paths(const struct checkout_opts *opts,
 
 	if (opts->patch_mode) {
 		enum add_p_mode patch_mode;
-		struct add_p_opt add_p_opt = {
+		struct interactive_options interactive_opts = {
 			.context = opts->patch_context,
 			.interhunkcontext = opts->patch_interhunk_context,
 			.auto_advance = opts->auto_advance
@@ -562,8 +562,8 @@ static int checkout_paths(const struct checkout_opts *opts,
 		else
 			BUG("either flag must have been set, worktree=%d, index=%d",
 			    opts->checkout_worktree, opts->checkout_index);
-		return !!run_add_p(the_repository, patch_mode, &add_p_opt,
-				   rev, &opts->pathspec);
+		return !!run_add_p(the_repository, patch_mode, &interactive_opts,
+				   rev, &opts->pathspec, 0);
 	}
 
 	repo_hold_locked_index(the_repository, &lock_file, LOCK_DIE_ON_ERROR);
@@ -891,7 +891,8 @@ static int merge_working_tree(const struct checkout_opts *opts,
 					0, 0);
 			init_ui_merge_options(&o, the_repository);
 			o.verbosity = 0;
-			work = write_in_core_index_as_tree(the_repository);
+			work = write_in_core_index_as_tree(the_repository,
+							   the_repository->index);
 
 			ret = reset_tree(new_tree,
 					 opts, 1,
diff --git a/builtin/commit.c b/builtin/commit.c
index 02a04f68be..a3e52ac9ca 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -123,7 +123,7 @@ static const char *edit_message, *use_message;
 static char *fixup_message, *fixup_commit, *squash_message;
 static const char *fixup_prefix;
 static int all, also, interactive, patch_interactive, only, amend, signoff;
-static struct add_p_opt add_p_opt = ADD_P_OPT_INIT;
+static struct interactive_options interactive_opts = INTERACTIVE_OPTIONS_INIT;
 static int edit_flag = -1; /* unspecified */
 static int quiet, verbose, no_verify, allow_empty, dry_run, renew_authorship;
 static int config_commit_verbose = -1; /* unspecified */
@@ -357,9 +357,9 @@ static const char *prepare_index(const char **argv, const char *prefix,
 	const char *ret;
 	char *path = NULL;
 
-	if (add_p_opt.context < -1)
+	if (interactive_opts.context < -1)
 		die(_("'%s' cannot be negative"), "--unified");
-	if (add_p_opt.interhunkcontext < -1)
+	if (interactive_opts.interhunkcontext < -1)
 		die(_("'%s' cannot be negative"), "--inter-hunk-context");
 
 	if (is_status)
@@ -408,7 +408,7 @@ static const char *prepare_index(const char **argv, const char *prefix,
 		old_index_env = xstrdup_or_null(getenv(INDEX_ENVIRONMENT));
 		setenv(INDEX_ENVIRONMENT, the_repository->index_file, 1);
 
-		if (interactive_add(the_repository, argv, prefix, patch_interactive, &add_p_opt) != 0)
+		if (interactive_add(the_repository, argv, prefix, patch_interactive, &interactive_opts) != 0)
 			die(_("interactive add failed"));
 
 		the_repository->index_file = old_repo_index_file;
@@ -433,9 +433,9 @@ static const char *prepare_index(const char **argv, const char *prefix,
 		ret = get_lock_file_path(&index_lock);
 		goto out;
 	} else {
-		if (add_p_opt.context != -1)
+		if (interactive_opts.context != -1)
 			die(_("the option '%s' requires '%s'"), "--unified", "--interactive/--patch");
-		if (add_p_opt.interhunkcontext != -1)
+		if (interactive_opts.interhunkcontext != -1)
 			die(_("the option '%s' requires '%s'"), "--inter-hunk-context", "--interactive/--patch");
 	}
 
@@ -1744,8 +1744,8 @@ int cmd_commit(int argc,
 		OPT_BOOL('i', "include", &also, N_("add specified files to index for commit")),
 		OPT_BOOL(0, "interactive", &interactive, N_("interactively add files")),
 		OPT_BOOL('p', "patch", &patch_interactive, N_("interactively add changes")),
-		OPT_DIFF_UNIFIED(&add_p_opt.context),
-		OPT_DIFF_INTERHUNK_CONTEXT(&add_p_opt.interhunkcontext),
+		OPT_DIFF_UNIFIED(&interactive_opts.context),
+		OPT_DIFF_INTERHUNK_CONTEXT(&interactive_opts.interhunkcontext),
 		OPT_BOOL('o', "only", &only, N_("commit only specified files")),
 		OPT_BOOL('n', "no-verify", &no_verify, N_("bypass pre-commit and commit-msg hooks")),
 		OPT_BOOL(0, "dry-run", &dry_run, N_("show what would be committed")),
diff --git a/builtin/history.c b/builtin/history.c
index 88822a184f..568dc75ee7 100644
--- a/builtin/history.c
+++ b/builtin/history.c
@@ -1,6 +1,7 @@
 #define USE_THE_REPOSITORY_VARIABLE
 
 #include "builtin.h"
+#include "cache-tree.h"
 #include "commit.h"
 #include "commit-reach.h"
 #include "config.h"
@@ -8,17 +9,24 @@
 #include "environment.h"
 #include "gettext.h"
 #include "hex.h"
+#include "lockfile.h"
+#include "oidmap.h"
 #include "parse-options.h"
+#include "path.h"
+#include "read-cache.h"
 #include "refs.h"
 #include "replay.h"
 #include "revision.h"
 #include "sequencer.h"
 #include "strvec.h"
 #include "tree.h"
+#include "unpack-trees.h"
 #include "wt-status.h"
 
 #define GIT_HISTORY_REWORD_USAGE \
 	N_("git history reword <commit> [--dry-run] [--update-refs=(branches|head)]")
+#define GIT_HISTORY_SPLIT_USAGE \
+	N_("git history split <commit> [--dry-run] [--update-refs=(branches|head)] [--] [<pathspec>...]")
 
 static void change_data_free(void *util, const char *str UNUSED)
 {
@@ -83,10 +91,13 @@ static int fill_commit_message(struct repository *repo,
 	return 0;
 }
 
-static int commit_tree_with_edited_message(struct repository *repo,
-					   const char *action,
-					   struct commit *original,
-					   struct commit **out)
+static int commit_tree_with_edited_message_ext(struct repository *repo,
+					       const char *action,
+					       struct commit *commit_with_message,
+					       const struct commit_list *parents,
+					       const struct object_id *old_tree,
+					       const struct object_id *new_tree,
+					       struct commit **out)
 {
 	const char *exclude_gpgsig[] = {
 		/* We reencode the message, so the encoding needs to be stripped. */
@@ -100,44 +111,27 @@ static int commit_tree_with_edited_message(struct repository *repo,
 	struct commit_extra_header *original_extra_headers = NULL;
 	struct strbuf commit_message = STRBUF_INIT;
 	struct object_id rewritten_commit_oid;
-	struct object_id original_tree_oid;
-	struct object_id parent_tree_oid;
 	char *original_author = NULL;
-	struct commit *parent;
 	size_t len;
 	int ret;
 
-	original_tree_oid = repo_get_commit_tree(repo, original)->object.oid;
-
-	parent = original->parents ? original->parents->item : NULL;
-	if (parent) {
-		if (repo_parse_commit(repo, parent)) {
-			ret = error(_("unable to parse parent commit %s"),
-				    oid_to_hex(&parent->object.oid));
-			goto out;
-		}
-
-		parent_tree_oid = repo_get_commit_tree(repo, parent)->object.oid;
-	} else {
-		oidcpy(&parent_tree_oid, repo->hash_algo->empty_tree);
-	}
-
 	/* We retain authorship of the original commit. */
-	original_message = repo_logmsg_reencode(repo, original, NULL, NULL);
+	original_message = repo_logmsg_reencode(repo, commit_with_message, NULL, NULL);
 	ptr = find_commit_header(original_message, "author", &len);
 	if (ptr)
 		original_author = xmemdupz(ptr, len);
 	find_commit_subject(original_message, &original_body);
 
-	ret = fill_commit_message(repo, &parent_tree_oid, &original_tree_oid,
+	ret = fill_commit_message(repo, old_tree, new_tree,
 				  original_body, action, &commit_message);
 	if (ret < 0)
 		goto out;
 
-	original_extra_headers = read_commit_extra_headers(original, exclude_gpgsig);
+	original_extra_headers = read_commit_extra_headers(commit_with_message,
+							   exclude_gpgsig);
 
-	ret = commit_tree_extended(commit_message.buf, commit_message.len, &original_tree_oid,
-				   original->parents, &rewritten_commit_oid, original_author,
+	ret = commit_tree_extended(commit_message.buf, commit_message.len, new_tree,
+				   parents, &rewritten_commit_oid, original_author,
 				   NULL, NULL, original_extra_headers);
 	if (ret < 0)
 		goto out;
@@ -151,6 +145,33 @@ out:
 	return ret;
 }
 
+static int commit_tree_with_edited_message(struct repository *repo,
+					   const char *action,
+					   struct commit *original,
+					   struct commit **out)
+{
+	struct object_id parent_tree_oid;
+	const struct object_id *tree_oid;
+	struct commit *parent;
+
+	tree_oid = &repo_get_commit_tree(repo, original)->object.oid;
+
+	parent = original->parents ? original->parents->item : NULL;
+	if (parent) {
+		if (repo_parse_commit(repo, parent)) {
+			return error(_("unable to parse parent commit %s"),
+				     oid_to_hex(&parent->object.oid));
+		}
+
+		parent_tree_oid = repo_get_commit_tree(repo, parent)->object.oid;
+	} else {
+		oidcpy(&parent_tree_oid, repo->hash_algo->empty_tree);
+	}
+
+	return commit_tree_with_edited_message_ext(repo, action, original, original->parents,
+						   &parent_tree_oid, tree_oid, out);
+}
+
 enum ref_action {
 	REF_ACTION_DEFAULT,
 	REF_ACTION_BRANCHES,
@@ -471,6 +492,246 @@ out:
 	return ret;
 }
 
+static int write_ondisk_index(struct repository *repo,
+			      struct object_id *oid,
+			      const char *path)
+{
+	struct unpack_trees_options opts = { 0 };
+	struct lock_file lock = LOCK_INIT;
+	struct tree_desc tree_desc;
+	struct index_state index;
+	struct tree *tree;
+	int ret;
+
+	index_state_init(&index, repo);
+
+	opts.head_idx = -1;
+	opts.src_index = &index;
+	opts.dst_index = &index;
+
+	tree = repo_parse_tree_indirect(repo, oid);
+	init_tree_desc(&tree_desc, &tree->object.oid, tree->buffer, tree->size);
+
+	if (unpack_trees(1, &tree_desc, &opts)) {
+		ret = error(_("unable to populate index with tree"));
+		goto out;
+	}
+
+	prime_cache_tree(repo, &index, tree);
+
+	if (hold_lock_file_for_update(&lock, path, 0) < 0) {
+		ret = error_errno(_("unable to acquire index lock"));
+		goto out;
+	}
+
+	if (write_locked_index(&index, &lock, COMMIT_LOCK)) {
+		ret = error(_("unable to write new index file"));
+		goto out;
+	}
+
+	ret = 0;
+
+out:
+	rollback_lock_file(&lock);
+	release_index(&index);
+	return ret;
+}
+
+static int split_commit(struct repository *repo,
+			struct commit *original,
+			struct pathspec *pathspec,
+			struct commit **out)
+{
+	struct interactive_options interactive_opts = INTERACTIVE_OPTIONS_INIT;
+	struct strbuf index_file = STRBUF_INIT;
+	struct index_state index = INDEX_STATE_INIT(repo);
+	const struct object_id *original_commit_tree_oid;
+	const struct object_id *old_tree_oid, *new_tree_oid;
+	struct object_id parent_tree_oid;
+	char original_commit_oid[GIT_MAX_HEXSZ + 1];
+	struct commit *first_commit, *second_commit;
+	struct commit_list *parents = NULL;
+	struct tree *split_tree;
+	int ret;
+
+	if (original->parents) {
+		if (repo_parse_commit(repo, original->parents->item)) {
+			ret = error(_("unable to parse parent commit %s"),
+				    oid_to_hex(&original->parents->item->object.oid));
+			goto out;
+		}
+
+		parent_tree_oid = *get_commit_tree_oid(original->parents->item);
+	} else {
+		oidcpy(&parent_tree_oid, repo->hash_algo->empty_tree);
+	}
+	original_commit_tree_oid = get_commit_tree_oid(original);
+
+	/*
+	 * Construct the first commit. This is done by taking the original
+	 * commit parent's tree and selectively patching changes from the diff
+	 * between that parent and its child.
+	 */
+	repo_git_path_replace(repo, &index_file, "%s", "history-split.index");
+
+	ret = write_ondisk_index(repo, &parent_tree_oid, index_file.buf);
+	if (ret < 0)
+		goto out;
+
+	ret = read_index_from(&index, index_file.buf, repo->gitdir);
+	if (ret < 0) {
+		ret = error(_("failed reading temporary index"));
+		goto out;
+	}
+
+	oid_to_hex_r(original_commit_oid, &original->object.oid);
+	ret = run_add_p_index(repo, &index, index_file.buf, &interactive_opts,
+			      original_commit_oid, pathspec, ADD_P_DISALLOW_EDIT);
+	if (ret < 0)
+		goto out;
+
+	split_tree = write_in_core_index_as_tree(repo, &index);
+	if (!split_tree) {
+		ret = error(_("failed split tree"));
+		goto out;
+	}
+
+	unlink(index_file.buf);
+	strbuf_release(&index_file);
+
+	/*
+	 * We disallow the cases where either the split-out commit or the
+	 * original commit would become empty. Consequently, if we see that the
+	 * new tree ID matches either of those trees we abort.
+	 */
+	if (oideq(&split_tree->object.oid, &parent_tree_oid)) {
+		ret = error(_("split commit is empty"));
+		goto out;
+	} else if (oideq(&split_tree->object.oid, original_commit_tree_oid)) {
+		ret = error(_("split commit tree matches original commit"));
+		goto out;
+	}
+
+	/*
+	 * The first commit is constructed from the split-out tree. The base
+	 * that shall be diffed against is the parent of the original commit.
+	 */
+	ret = commit_tree_with_edited_message_ext(repo, "split-out", original,
+						  original->parents, &parent_tree_oid,
+						  &split_tree->object.oid, &first_commit);
+	if (ret < 0) {
+		ret = error(_("failed writing first commit"));
+		goto out;
+	}
+
+	/*
+	 * The second commit is constructed from the original tree. The base to
+	 * diff against and the parent in this case is the first split-out
+	 * commit.
+	 */
+	commit_list_append(first_commit, &parents);
+
+	old_tree_oid = &repo_get_commit_tree(repo, first_commit)->object.oid;
+	new_tree_oid = &repo_get_commit_tree(repo, original)->object.oid;
+
+	ret = commit_tree_with_edited_message_ext(repo, "split-out", original,
+						  parents, old_tree_oid,
+						  new_tree_oid, &second_commit);
+	if (ret < 0) {
+		ret = error(_("failed writing second commit"));
+		goto out;
+	}
+
+	*out = second_commit;
+	ret = 0;
+
+out:
+	if (index_file.len)
+		unlink(index_file.buf);
+	strbuf_release(&index_file);
+	free_commit_list(parents);
+	release_index(&index);
+	return ret;
+}
+
+static int cmd_history_split(int argc,
+			     const char **argv,
+			     const char *prefix,
+			     struct repository *repo)
+{
+	const char * const usage[] = {
+		GIT_HISTORY_SPLIT_USAGE,
+		NULL,
+	};
+	enum ref_action action = REF_ACTION_DEFAULT;
+	int dry_run = 0;
+	struct option options[] = {
+		OPT_CALLBACK_F(0, "update-refs", &action, N_("<refs>"),
+			       N_("control ref update behavior (branches|head|print)"),
+			       PARSE_OPT_NONEG, parse_ref_action),
+		OPT_BOOL('n', "dry-run", &dry_run,
+			 N_("perform a dry-run without updating any refs")),
+		OPT_END(),
+	};
+	struct commit *original, *rewritten = NULL;
+	struct strbuf reflog_msg = STRBUF_INIT;
+	struct pathspec pathspec = { 0 };
+	struct rev_info revs = { 0 };
+	int ret;
+
+	argc = parse_options(argc, argv, prefix, options, usage, 0);
+	if (argc < 1) {
+		ret = error(_("command expects a committish"));
+		goto out;
+	}
+	repo_config(repo, git_default_config, NULL);
+
+	if (action == REF_ACTION_DEFAULT)
+		action = REF_ACTION_BRANCHES;
+
+	parse_pathspec(&pathspec, 0,
+		       PATHSPEC_PREFER_FULL |
+		       PATHSPEC_SYMLINK_LEADING_PATH |
+		       PATHSPEC_PREFIX_ORIGIN,
+		       prefix, argv + 1);
+
+	original = lookup_commit_reference_by_name(argv[0]);
+	if (!original) {
+		ret = error(_("commit cannot be found: %s"), argv[0]);
+		goto out;
+	}
+
+	ret = setup_revwalk(repo, action, original, &revs);
+	if (ret < 0)
+		goto out;
+
+	if (original->parents && original->parents->next) {
+		ret = error(_("cannot split up merge commit"));
+		goto out;
+	}
+
+	ret = split_commit(repo, original, &pathspec, &rewritten);
+	if (ret < 0)
+		goto out;
+
+	strbuf_addf(&reflog_msg, "split: updating %s", argv[0]);
+
+	ret = handle_reference_updates(&revs, action, original, rewritten,
+				       reflog_msg.buf, dry_run);
+	if (ret < 0) {
+		ret = error(_("failed replaying descendants"));
+		goto out;
+	}
+
+	ret = 0;
+
+out:
+	strbuf_release(&reflog_msg);
+	clear_pathspec(&pathspec);
+	release_revisions(&revs);
+	return ret;
+}
+
 int cmd_history(int argc,
 		const char **argv,
 		const char *prefix,
@@ -478,11 +739,13 @@ int cmd_history(int argc,
 {
 	const char * const usage[] = {
 		GIT_HISTORY_REWORD_USAGE,
+		GIT_HISTORY_SPLIT_USAGE,
 		NULL,
 	};
 	parse_opt_subcommand_fn *fn = NULL;
 	struct option options[] = {
 		OPT_SUBCOMMAND("reword", &fn, cmd_history_reword),
+		OPT_SUBCOMMAND("split", &fn, cmd_history_split),
 		OPT_END(),
 	};
 
diff --git a/builtin/reset.c b/builtin/reset.c
index 88f95f9fc7..3590be57a5 100644
--- a/builtin/reset.c
+++ b/builtin/reset.c
@@ -346,7 +346,7 @@ int cmd_reset(int argc,
 	struct object_id oid;
 	struct pathspec pathspec;
 	int intent_to_add = 0;
-	struct add_p_opt add_p_opt = ADD_P_OPT_INIT;
+	struct interactive_options interactive_opts = INTERACTIVE_OPTIONS_INIT;
 	const struct option options[] = {
 		OPT__QUIET(&quiet, N_("be quiet, only report errors")),
 		OPT_BOOL(0, "no-refresh", &no_refresh,
@@ -371,10 +371,10 @@ int cmd_reset(int argc,
 			       PARSE_OPT_OPTARG,
 			       option_parse_recurse_submodules_worktree_updater),
 		OPT_BOOL('p', "patch", &patch_mode, N_("select hunks interactively")),
-		OPT_BOOL(0, "auto-advance", &add_p_opt.auto_advance,
+		OPT_BOOL(0, "auto-advance", &interactive_opts.auto_advance,
 			 N_("auto advance to the next file when selecting hunks interactively")),
-		OPT_DIFF_UNIFIED(&add_p_opt.context),
-		OPT_DIFF_INTERHUNK_CONTEXT(&add_p_opt.interhunkcontext),
+		OPT_DIFF_UNIFIED(&interactive_opts.context),
+		OPT_DIFF_INTERHUNK_CONTEXT(&interactive_opts.interhunkcontext),
 		OPT_BOOL('N', "intent-to-add", &intent_to_add,
 				N_("record only the fact that removed paths will be added later")),
 		OPT_PATHSPEC_FROM_FILE(&pathspec_from_file),
@@ -425,9 +425,9 @@ int cmd_reset(int argc,
 		oidcpy(&oid, &tree->object.oid);
 	}
 
-	if (add_p_opt.context < -1)
+	if (interactive_opts.context < -1)
 		die(_("'%s' cannot be negative"), "--unified");
-	if (add_p_opt.interhunkcontext < -1)
+	if (interactive_opts.interhunkcontext < -1)
 		die(_("'%s' cannot be negative"), "--inter-hunk-context");
 
 	prepare_repo_settings(the_repository);
@@ -438,14 +438,14 @@ int cmd_reset(int argc,
 			die(_("options '%s' and '%s' cannot be used together"), "--patch", "--{hard,mixed,soft}");
 		trace2_cmd_mode("patch-interactive");
 		update_ref_status = !!run_add_p(the_repository, ADD_P_RESET,
-						&add_p_opt, rev, &pathspec);
+						&interactive_opts, rev, &pathspec, 0);
 		goto cleanup;
 	} else {
-		if (add_p_opt.context != -1)
+		if (interactive_opts.context != -1)
 			die(_("the option '%s' requires '%s'"), "--unified", "--patch");
-		if (add_p_opt.interhunkcontext != -1)
+		if (interactive_opts.interhunkcontext != -1)
 			die(_("the option '%s' requires '%s'"), "--inter-hunk-context", "--patch");
-		if (!add_p_opt.auto_advance)
+		if (!interactive_opts.auto_advance)
 			die(_("the option '%s' requires '%s'"), "--no-auto-advance", "--patch");
 	}
 
diff --git a/builtin/stash.c b/builtin/stash.c
index e79d612e57..7c68a1d7f9 100644
--- a/builtin/stash.c
+++ b/builtin/stash.c
@@ -1306,7 +1306,7 @@ done:
 
 static int stash_patch(struct stash_info *info, const struct pathspec *ps,
 		       struct strbuf *out_patch, int quiet,
-		       struct add_p_opt *add_p_opt)
+		       struct interactive_options *interactive_opts)
 {
 	int ret = 0;
 	struct child_process cp_read_tree = CHILD_PROCESS_INIT;
@@ -1331,7 +1331,7 @@ static int stash_patch(struct stash_info *info, const struct pathspec *ps,
 	old_index_env = xstrdup_or_null(getenv(INDEX_ENVIRONMENT));
 	setenv(INDEX_ENVIRONMENT, the_repository->index_file, 1);
 
-	ret = !!run_add_p(the_repository, ADD_P_STASH, add_p_opt, NULL, ps);
+	ret = !!run_add_p(the_repository, ADD_P_STASH, interactive_opts, NULL, ps, 0);
 
 	the_repository->index_file = old_repo_index_file;
 	if (old_index_env && *old_index_env)
@@ -1427,7 +1427,8 @@ done:
 }
 
 static int do_create_stash(const struct pathspec *ps, struct strbuf *stash_msg_buf,
-			   int include_untracked, int patch_mode, struct add_p_opt *add_p_opt,
+			   int include_untracked, int patch_mode,
+			   struct interactive_options *interactive_opts,
 			   int only_staged, struct stash_info *info, struct strbuf *patch,
 			   int quiet)
 {
@@ -1509,7 +1510,7 @@ static int do_create_stash(const struct pathspec *ps, struct strbuf *stash_msg_b
 		untracked_commit_option = 1;
 	}
 	if (patch_mode) {
-		ret = stash_patch(info, ps, patch, quiet, add_p_opt);
+		ret = stash_patch(info, ps, patch, quiet, interactive_opts);
 		if (ret < 0) {
 			if (!quiet)
 				fprintf_ln(stderr, _("Cannot save the current "
@@ -1595,7 +1596,8 @@ static int create_stash(int argc, const char **argv, const char *prefix UNUSED,
 }
 
 static int do_push_stash(const struct pathspec *ps, const char *stash_msg, int quiet,
-			 int keep_index, int patch_mode, struct add_p_opt *add_p_opt,
+			 int keep_index, int patch_mode,
+			 struct interactive_options *interactive_opts,
 			 int include_untracked, int only_staged)
 {
 	int ret = 0;
@@ -1667,7 +1669,7 @@ static int do_push_stash(const struct pathspec *ps, const char *stash_msg, int q
 	if (stash_msg)
 		strbuf_addstr(&stash_msg_buf, stash_msg);
 	if (do_create_stash(ps, &stash_msg_buf, include_untracked, patch_mode,
-			    add_p_opt, only_staged, &info, &patch, quiet)) {
+			    interactive_opts, only_staged, &info, &patch, quiet)) {
 		ret = -1;
 		goto done;
 	}
@@ -1841,7 +1843,7 @@ static int push_stash(int argc, const char **argv, const char *prefix,
 	const char *stash_msg = NULL;
 	char *pathspec_from_file = NULL;
 	struct pathspec ps;
-	struct add_p_opt add_p_opt = ADD_P_OPT_INIT;
+	struct interactive_options interactive_opts = INTERACTIVE_OPTIONS_INIT;
 	struct option options[] = {
 		OPT_BOOL('k', "keep-index", &keep_index,
 			 N_("keep index")),
@@ -1849,10 +1851,10 @@ static int push_stash(int argc, const char **argv, const char *prefix,
 			 N_("stash staged changes only")),
 		OPT_BOOL('p', "patch", &patch_mode,
 			 N_("stash in patch mode")),
-		OPT_BOOL(0, "auto-advance", &add_p_opt.auto_advance,
+		OPT_BOOL(0, "auto-advance", &interactive_opts.auto_advance,
 			 N_("auto advance to the next file when selecting hunks interactively")),
-		OPT_DIFF_UNIFIED(&add_p_opt.context),
-		OPT_DIFF_INTERHUNK_CONTEXT(&add_p_opt.interhunkcontext),
+		OPT_DIFF_UNIFIED(&interactive_opts.context),
+		OPT_DIFF_INTERHUNK_CONTEXT(&interactive_opts.interhunkcontext),
 		OPT__QUIET(&quiet, N_("quiet mode")),
 		OPT_BOOL('u', "include-untracked", &include_untracked,
 			 N_("include untracked files in stash")),
@@ -1909,21 +1911,21 @@ static int push_stash(int argc, const char **argv, const char *prefix,
 	}
 
 	if (!patch_mode) {
-		if (add_p_opt.context != -1)
+		if (interactive_opts.context != -1)
 			die(_("the option '%s' requires '%s'"), "--unified", "--patch");
-		if (add_p_opt.interhunkcontext != -1)
+		if (interactive_opts.interhunkcontext != -1)
 			die(_("the option '%s' requires '%s'"), "--inter-hunk-context", "--patch");
-		if (!add_p_opt.auto_advance)
+		if (!interactive_opts.auto_advance)
 			die(_("the option '%s' requires '%s'"), "--no-auto-advance", "--patch");
 	}
 
-	if (add_p_opt.context < -1)
+	if (interactive_opts.context < -1)
 		die(_("'%s' cannot be negative"), "--unified");
-	if (add_p_opt.interhunkcontext < -1)
+	if (interactive_opts.interhunkcontext < -1)
 		die(_("'%s' cannot be negative"), "--inter-hunk-context");
 
 	ret = do_push_stash(&ps, stash_msg, quiet, keep_index, patch_mode,
-			    &add_p_opt, include_untracked, only_staged);
+			    &interactive_opts, include_untracked, only_staged);
 
 	clear_pathspec(&ps);
 	free(pathspec_from_file);
@@ -1948,7 +1950,7 @@ static int save_stash(int argc, const char **argv, const char *prefix,
 	const char *stash_msg = NULL;
 	struct pathspec ps;
 	struct strbuf stash_msg_buf = STRBUF_INIT;
-	struct add_p_opt add_p_opt = ADD_P_OPT_INIT;
+	struct interactive_options interactive_opts = INTERACTIVE_OPTIONS_INIT;
 	struct option options[] = {
 		OPT_BOOL('k', "keep-index", &keep_index,
 			 N_("keep index")),
@@ -1956,10 +1958,10 @@ static int save_stash(int argc, const char **argv, const char *prefix,
 			 N_("stash staged changes only")),
 		OPT_BOOL('p', "patch", &patch_mode,
 			 N_("stash in patch mode")),
-		OPT_BOOL(0, "auto-advance", &add_p_opt.auto_advance,
+		OPT_BOOL(0, "auto-advance", &interactive_opts.auto_advance,
 			 N_("auto advance to the next file when selecting hunks interactively")),
-		OPT_DIFF_UNIFIED(&add_p_opt.context),
-		OPT_DIFF_INTERHUNK_CONTEXT(&add_p_opt.interhunkcontext),
+		OPT_DIFF_UNIFIED(&interactive_opts.context),
+		OPT_DIFF_INTERHUNK_CONTEXT(&interactive_opts.interhunkcontext),
 		OPT__QUIET(&quiet, N_("quiet mode")),
 		OPT_BOOL('u', "include-untracked", &include_untracked,
 			 N_("include untracked files in stash")),
@@ -1979,22 +1981,22 @@ static int save_stash(int argc, const char **argv, const char *prefix,
 
 	memset(&ps, 0, sizeof(ps));
 
-	if (add_p_opt.context < -1)
+	if (interactive_opts.context < -1)
 		die(_("'%s' cannot be negative"), "--unified");
-	if (add_p_opt.interhunkcontext < -1)
+	if (interactive_opts.interhunkcontext < -1)
 		die(_("'%s' cannot be negative"), "--inter-hunk-context");
 
 	if (!patch_mode) {
-		if (add_p_opt.context != -1)
+		if (interactive_opts.context != -1)
 			die(_("the option '%s' requires '%s'"), "--unified", "--patch");
-		if (add_p_opt.interhunkcontext != -1)
+		if (interactive_opts.interhunkcontext != -1)
 			die(_("the option '%s' requires '%s'"), "--inter-hunk-context", "--patch");
-		if (!add_p_opt.auto_advance)
+		if (!interactive_opts.auto_advance)
 			die(_("the option '%s' requires '%s'"), "--no-auto-advance", "--patch");
 	}
 
 	ret = do_push_stash(&ps, stash_msg, quiet, keep_index,
-			    patch_mode, &add_p_opt, include_untracked,
+			    patch_mode, &interactive_opts, include_untracked,
 			    only_staged);
 
 	strbuf_release(&stash_msg_buf);
diff --git a/cache-tree.c b/cache-tree.c
index 16c3a36b48..60bcc07c3b 100644
--- a/cache-tree.c
+++ b/cache-tree.c
@@ -723,11 +723,11 @@ static int write_index_as_tree_internal(struct object_id *oid,
 	return 0;
 }
 
-struct tree* write_in_core_index_as_tree(struct repository *repo) {
+struct tree *write_in_core_index_as_tree(struct repository *repo,
+					 struct index_state *index_state) {
 	struct object_id o;
 	int was_valid, ret;
 
-	struct index_state *index_state	= repo->index;
 	was_valid = index_state->cache_tree &&
 		    cache_tree_fully_valid(index_state->cache_tree);
 
diff --git a/cache-tree.h b/cache-tree.h
index b82c4963e7..f8bddae523 100644
--- a/cache-tree.h
+++ b/cache-tree.h
@@ -47,7 +47,8 @@ int cache_tree_verify(struct repository *, struct index_state *);
 #define WRITE_TREE_UNMERGED_INDEX (-2)
 #define WRITE_TREE_PREFIX_ERROR (-3)
 
-struct tree* write_in_core_index_as_tree(struct repository *repo);
+struct tree *write_in_core_index_as_tree(struct repository *repo,
+					 struct index_state *index_state);
 int write_index_as_tree(struct object_id *oid, struct index_state *index_state, const char *index_path, int flags, const char *prefix);
 void prime_cache_tree(struct repository *, struct index_state *, struct tree *);
 
diff --git a/commit.h b/commit.h
index e9dee8d26b..58150045af 100644
--- a/commit.h
+++ b/commit.h
@@ -287,7 +287,7 @@ int for_each_commit_graft(each_commit_graft_fn, void *);
 int interactive_add(struct repository *repo,
 		    const char **argv,
 		    const char *prefix,
-		    int patch, struct add_p_opt *add_p_opt);
+		    int patch, struct interactive_options *opts);
 
 struct commit_extra_header {
 	struct commit_extra_header *next;
diff --git a/t/meson.build b/t/meson.build
index 026bf02fd6..8757b29595 100644
--- a/t/meson.build
+++ b/t/meson.build
@@ -396,6 +396,7 @@ integration_tests = [
   't3440-rebase-trailer.sh',
   't3450-history.sh',
   't3451-history-reword.sh',
+  't3452-history-split.sh',
   't3500-cherry.sh',
   't3501-revert-cherry-pick.sh',
   't3502-cherry-pick-merge.sh',
diff --git a/t/t3452-history-split.sh b/t/t3452-history-split.sh
new file mode 100755
index 0000000000..8ed0cebb50
--- /dev/null
+++ b/t/t3452-history-split.sh
@@ -0,0 +1,757 @@
+#!/bin/sh
+
+test_description='tests for git-history split subcommand'
+
+. ./test-lib.sh
+. "$TEST_DIRECTORY/lib-log-graph.sh"
+
+# The fake editor takes multiple arguments, each of which represents a commit
+# message. Subsequent invocations of the editor will then yield those messages
+# in order.
+#
+set_fake_editor () {
+	printf "%s\n" "$@" >fake-input &&
+	write_script fake-editor.sh <<-\EOF &&
+	head -n1 fake-input >"$1"
+	sed 1d fake-input >fake-input.trimmed &&
+	mv fake-input.trimmed fake-input
+	EOF
+	test_set_editor "$(pwd)"/fake-editor.sh
+}
+
+expect_graph () {
+	cat >expect &&
+	lib_test_cmp_graph --graph --format=%s "$@"
+}
+
+expect_log () {
+	git log --format="%s" >actual &&
+	cat >expect &&
+	test_cmp expect actual
+}
+
+expect_tree_entries () {
+	git ls-tree --name-only "$1" >actual &&
+	cat >expect &&
+	test_cmp expect actual
+}
+
+test_expect_success 'refuses to work with merge commits' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		test_commit base &&
+		git branch branch &&
+		test_commit ours &&
+		git switch branch &&
+		test_commit theirs &&
+		git switch - &&
+		git merge theirs &&
+		test_must_fail git history split HEAD 2>err &&
+		test_grep "cannot split up merge commit" err &&
+		test_must_fail git history split HEAD~ 2>err &&
+		test_grep "replaying merge commits is not supported yet" err
+	)
+'
+
+test_expect_success 'errors on missing commit argument' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		test_commit initial &&
+		test_must_fail git history split 2>err &&
+		test_grep "command expects a committish" err
+	)
+'
+
+test_expect_success 'errors on unknown revision' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		test_commit initial &&
+		test_must_fail git history split does-not-exist 2>err &&
+		test_grep "commit cannot be found" err
+	)
+'
+
+test_expect_success '--dry-run does not modify any refs' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		test_commit base &&
+		touch bar foo &&
+		git add . &&
+		git commit -m split-me &&
+
+		git refs list --include-root-refs >before &&
+
+		set_fake_editor "first" "second" &&
+		git history split --dry-run HEAD <<-EOF &&
+		y
+		n
+		EOF
+
+		git refs list --include-root-refs >after &&
+		test_cmp before after
+	)
+'
+
+test_expect_success 'can split up tip commit' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		test_commit initial &&
+		touch bar foo &&
+		git add . &&
+		git commit -m split-me &&
+
+		git symbolic-ref HEAD >expect &&
+		set_fake_editor "first" "second" &&
+		git history split HEAD <<-EOF &&
+		y
+		n
+		EOF
+		git symbolic-ref HEAD >actual &&
+		test_cmp expect actual &&
+
+		expect_log <<-EOF &&
+		second
+		first
+		initial
+		EOF
+
+		expect_tree_entries HEAD~ <<-EOF &&
+		bar
+		initial.t
+		EOF
+
+		expect_tree_entries HEAD <<-EOF &&
+		bar
+		foo
+		initial.t
+		EOF
+
+		git reflog >reflog &&
+		test_grep "split: updating HEAD" reflog
+	)
+'
+
+test_expect_success 'can split up root commit' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		touch bar foo &&
+		git add . &&
+		git commit -m root &&
+		test_commit tip &&
+
+		set_fake_editor "first" "second" &&
+		git history split HEAD~ <<-EOF &&
+		y
+		n
+		EOF
+
+		expect_log <<-EOF &&
+		tip
+		second
+		first
+		EOF
+
+		expect_tree_entries HEAD~2 <<-EOF &&
+		bar
+		EOF
+
+		expect_tree_entries HEAD~ <<-EOF &&
+		bar
+		foo
+		EOF
+
+		expect_tree_entries HEAD <<-EOF
+		bar
+		foo
+		tip.t
+		EOF
+	)
+'
+
+test_expect_success 'can split up in-between commit' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		test_commit initial &&
+		touch bar foo &&
+		git add . &&
+		git commit -m split-me &&
+		test_commit tip &&
+
+		set_fake_editor "first" "second" &&
+		git history split HEAD~ <<-EOF &&
+		y
+		n
+		EOF
+
+		expect_log <<-EOF &&
+		tip
+		second
+		first
+		initial
+		EOF
+
+		expect_tree_entries HEAD~2 <<-EOF &&
+		bar
+		initial.t
+		EOF
+
+		expect_tree_entries HEAD~ <<-EOF &&
+		bar
+		foo
+		initial.t
+		EOF
+
+		expect_tree_entries HEAD <<-EOF
+		bar
+		foo
+		initial.t
+		tip.t
+		EOF
+	)
+'
+
+test_expect_success 'can split HEAD only' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		test_commit base &&
+		touch a b &&
+		git add . &&
+		git commit -m split-me &&
+		git branch unrelated &&
+
+		set_fake_editor "ours-a" "ours-b" &&
+		git history split --update-refs=head HEAD <<-EOF &&
+		y
+		n
+		EOF
+		expect_graph --branches <<-EOF
+		* ours-b
+		* ours-a
+		| * split-me
+		|/
+		* base
+		EOF
+	)
+'
+
+test_expect_success 'can split detached HEAD' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		test_commit initial &&
+		touch bar foo &&
+		git add . &&
+		git commit -m split-me &&
+		git checkout --detach HEAD &&
+
+		set_fake_editor "first" "second" &&
+		git history split --update-refs=head HEAD <<-EOF &&
+		y
+		n
+		EOF
+
+		# HEAD should be detached and updated.
+		test_must_fail git symbolic-ref HEAD &&
+
+		expect_log <<-EOF
+		second
+		first
+		initial
+		EOF
+	)
+'
+
+test_expect_success 'can split commit in unrelated branch' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		test_commit base &&
+		git branch ours &&
+		git switch --create theirs &&
+		touch theirs-a theirs-b &&
+		git add . &&
+		git commit -m theirs &&
+		git switch ours &&
+		test_commit ours &&
+
+		# With --update-refs=head it is not possible to split up a
+		# commit that is unrelated to HEAD.
+		test_must_fail git history split --update-refs=head theirs 2>err &&
+		test_grep "rewritten commit must be an ancestor of HEAD" err &&
+
+		set_fake_editor "theirs-rewritten-a" "theirs-rewritten-b" &&
+		git history split theirs <<-EOF &&
+		y
+		n
+		EOF
+		expect_graph --branches <<-EOF &&
+		* ours
+		| * theirs-rewritten-b
+		| * theirs-rewritten-a
+		|/
+		* base
+		EOF
+
+		expect_tree_entries theirs~ <<-EOF &&
+		base.t
+		theirs-a
+		EOF
+
+		expect_tree_entries theirs <<-EOF
+		base.t
+		theirs-a
+		theirs-b
+		EOF
+	)
+'
+
+test_expect_success 'updates multiple descendant branches' '
+	test_when_finished "rm -rf repo" &&
+	git init repo --initial-branch=main &&
+	(
+		cd repo &&
+		test_commit base &&
+		touch file-a file-b &&
+		git add . &&
+		git commit -m split-me &&
+		git branch branch &&
+		test_commit on-main &&
+		git switch branch &&
+		test_commit on-branch &&
+		git switch main &&
+
+		set_fake_editor "split-a" "split-b" &&
+		git history split HEAD~ <<-EOF &&
+		y
+		n
+		EOF
+
+		# Both branches should now descend from the split commits.
+		expect_graph --branches <<-EOF
+		* on-branch
+		| * on-main
+		|/
+		* split-b
+		* split-a
+		* base
+		EOF
+	)
+'
+
+test_expect_success 'can pick multiple hunks' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		touch bar baz foo qux &&
+		git add . &&
+		git commit -m split-me &&
+
+		set_fake_editor "first" "second" &&
+		git history split HEAD <<-EOF &&
+		y
+		n
+		y
+		n
+		EOF
+
+		expect_tree_entries HEAD~ <<-EOF &&
+		bar
+		foo
+		EOF
+
+		expect_tree_entries HEAD <<-EOF
+		bar
+		baz
+		foo
+		qux
+		EOF
+	)
+'
+
+test_expect_success 'can use only last hunk' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		touch bar foo &&
+		git add . &&
+		git commit -m split-me &&
+
+		set_fake_editor "first" "second" &&
+		git history split HEAD <<-EOF &&
+		n
+		y
+		EOF
+
+		expect_log <<-EOF &&
+		second
+		first
+		EOF
+
+		expect_tree_entries HEAD~ <<-EOF &&
+		foo
+		EOF
+
+		expect_tree_entries HEAD <<-EOF
+		bar
+		foo
+		EOF
+	)
+'
+
+test_expect_success 'can split commit with file deletions' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		echo a >a &&
+		echo b >b &&
+		echo c >c &&
+		git add . &&
+		git commit -m base &&
+		git rm a b &&
+		git commit -m delete-both &&
+
+		set_fake_editor "delete-a" "delete-b" &&
+		git history split HEAD <<-EOF &&
+		y
+		n
+		EOF
+
+		expect_log <<-EOF &&
+		delete-b
+		delete-a
+		base
+		EOF
+
+		expect_tree_entries HEAD~ <<-EOF &&
+		b
+		c
+		EOF
+
+		expect_tree_entries HEAD <<-EOF
+		c
+		EOF
+	)
+'
+
+test_expect_success 'preserves original authorship' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		test_commit initial &&
+		touch bar foo &&
+		git add . &&
+		GIT_AUTHOR_NAME="Other Author" \
+		GIT_AUTHOR_EMAIL="other@example.com" \
+		git commit -m split-me &&
+
+		set_fake_editor "first" "second" &&
+		git history split HEAD <<-EOF &&
+		y
+		n
+		EOF
+
+		git log -1 --format="%an <%ae>" HEAD~ >actual &&
+		echo "Other Author <other@example.com>" >expect &&
+		test_cmp expect actual &&
+
+		git log -1 --format="%an <%ae>" HEAD >actual &&
+		test_cmp expect actual
+	)
+'
+
+test_expect_success 'aborts with empty commit message' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		touch bar foo &&
+		git add . &&
+		git commit -m split-me &&
+
+		set_fake_editor "" &&
+		test_must_fail git history split HEAD <<-EOF 2>err &&
+		y
+		n
+		EOF
+		test_grep "Aborting commit due to empty commit message." err
+	)
+'
+
+test_expect_success 'commit message editor sees split-out changes' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		touch bar foo &&
+		git add . &&
+		git commit -m split-me &&
+
+		write_script fake-editor.sh <<-\EOF &&
+		cat "$1" >>MESSAGES &&
+		echo "some commit message" >"$1"
+		EOF
+		test_set_editor "$(pwd)"/fake-editor.sh &&
+
+		git history split HEAD <<-EOF &&
+		y
+		n
+		EOF
+
+		# Note that we expect to see the messages twice, once for each
+		# of the commits. The committed files are different though.
+		cat >expect <<-EOF &&
+		split-me
+
+		# Please enter the commit message for the split-out changes. Lines starting
+		# with ${SQ}#${SQ} will be ignored, and an empty message aborts the commit.
+		# Changes to be committed:
+		#	new file:   bar
+		#
+		split-me
+
+		# Please enter the commit message for the split-out changes. Lines starting
+		# with ${SQ}#${SQ} will be ignored, and an empty message aborts the commit.
+		# Changes to be committed:
+		#	new file:   foo
+		#
+		EOF
+		test_cmp expect MESSAGES &&
+
+		expect_log <<-EOF
+		some commit message
+		some commit message
+		EOF
+	)
+'
+
+test_expect_success 'can use pathspec to limit what gets split' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		touch bar foo &&
+		git add . &&
+		git commit -m split-me &&
+
+		set_fake_editor "first" "second" &&
+		git history split HEAD -- foo <<-EOF &&
+		y
+		EOF
+
+		expect_tree_entries HEAD~ <<-EOF &&
+		foo
+		EOF
+
+		expect_tree_entries HEAD <<-EOF
+		bar
+		foo
+		EOF
+	)
+'
+
+test_expect_success 'pathspec matching no files produces empty split error' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		test_commit initial &&
+		touch bar foo &&
+		git add . &&
+		git commit -m split-me &&
+
+		set_fake_editor "first" "second" &&
+		test_must_fail git history split HEAD -- nonexistent 2>err &&
+		test_grep "split commit is empty" err
+	)
+'
+
+test_expect_success 'split with multiple pathspecs' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		test_commit initial &&
+		touch a b c d &&
+		git add . &&
+		git commit -m split-me &&
+
+		# Only a and c should be offered for splitting.
+		set_fake_editor "split-ac" "remainder" &&
+		git history split HEAD -- a c <<-EOF &&
+		y
+		y
+		EOF
+
+		expect_tree_entries HEAD~ <<-EOF &&
+		a
+		c
+		initial.t
+		EOF
+
+		expect_tree_entries HEAD <<-EOF
+		a
+		b
+		c
+		d
+		initial.t
+		EOF
+	)
+'
+
+test_expect_success 'split with file mode change' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		echo content >script &&
+		git add . &&
+		git commit -m base &&
+		test_chmod +x script &&
+		echo change >script &&
+		git commit -a -m "mode and content change" &&
+
+		set_fake_editor "mode-change" "content-change" &&
+		git history split HEAD <<-EOF &&
+		y
+		n
+		EOF
+
+		expect_log <<-EOF
+		content-change
+		mode-change
+		base
+		EOF
+	)
+'
+
+test_expect_success 'refuses to create empty split-out commit' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		test_commit base &&
+		touch bar foo &&
+		git add . &&
+		git commit -m split-me &&
+
+		test_must_fail git history split HEAD 2>err <<-EOF &&
+		n
+		n
+		EOF
+		test_grep "split commit is empty" err
+	)
+'
+
+test_expect_success 'hooks are not executed for rewritten commits' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		touch bar foo &&
+		git add . &&
+		git commit -m split-me &&
+		old_head=$(git rev-parse HEAD) &&
+
+		ORIG_PATH="$(pwd)" &&
+		export ORIG_PATH &&
+		for hook in prepare-commit-msg pre-commit post-commit post-rewrite commit-msg
+		do
+			write_script .git/hooks/$hook <<-\EOF || exit 1
+			touch "$ORIG_PATH"/hooks.log
+			EOF
+		done &&
+
+		set_fake_editor "first" "second" &&
+		git history split HEAD <<-EOF &&
+		y
+		n
+		EOF
+
+		expect_log <<-EOF &&
+		second
+		first
+		EOF
+
+		test_path_is_missing hooks.log
+	)
+'
+
+test_expect_success 'refuses to create empty original commit' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		touch bar foo &&
+		git add . &&
+		git commit -m split-me &&
+
+		test_must_fail git history split HEAD 2>err <<-EOF &&
+		y
+		y
+		EOF
+		test_grep "split commit tree matches original commit" err
+	)
+'
+
+test_expect_success 'retains changes in the worktree and index' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		echo a >a &&
+		echo b >b &&
+		git add . &&
+		git commit -m "initial commit" &&
+		echo a-modified >a &&
+		echo b-modified >b &&
+		git add b &&
+		set_fake_editor "a-only" "remainder" &&
+		git history split HEAD <<-EOF &&
+		y
+		n
+		EOF
+
+		expect_tree_entries HEAD~ <<-EOF &&
+		a
+		EOF
+		expect_tree_entries HEAD <<-EOF &&
+		a
+		b
+		EOF
+
+		cat >expect <<-\EOF &&
+		 M a
+		M  b
+		?? actual
+		?? expect
+		?? fake-editor.sh
+		?? fake-input
+		EOF
+		git status --porcelain >actual &&
+		test_cmp expect actual
+	)
+'
+
+test_done